Optical communication method, optical linking device and optical communication system

ABSTRACT

The system includes optical bus-bridging devices for observing the modes of said electric buses and the modes of said optical fibers while said electric buses have not been driven (OFF mode), so that the modes of the two electric buses connected through optical fibers are brought into agreement and that the buses can be simultaneously driven by a plurality of nodes. While one or both of said electric buses have been driven (ON mode) by the nodes connected thereto, an optical output has been continuously produced from the buses that are being driven to said optical fibers, and while light has been inputted from said optical fibers, the modes of said buses are not observed, but an electric output is produced to the electric bus of the side to which light is inputted to drive the bus. The optical bus-bridging device changes the mode of the electric bus when the optical fiber does not change within a predetermined period of time after the optical bus-bridging device has outputted a signal to the optical fiber.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an optical communication system and,particularly, to a system for bringing the states of buses intoagreement when the two electric buses in a network are connectedtogether through optical fibers.

A field LAN for industrial use is installed on a field and is subject tobe affected by electromagnetic noise from power cables and by lightning.If an optical fiber which is a noise-resistant transmission medium isused instead of an electric cable, therefore, it becomes possible tobury the power line and the control LAN in the same channel. Generally,however, the optical transmission devices are more expensive than theelectric transmission equipment. When the whole apparatus is connectedby using optical fibers, therefore, the system cost is driven up. On theother hand, the optical transmission equipment is used only in limitedplaces in the system. Therefore, if the places where expensive opticaltransmission devices are used are limited, then, the cost of the systemcan be suppressed.

To meet this demand, a constitution has been put into practical use asan optical linking device (LWZ440) for a program controller (S10/2 α) inour company (Hitachi, Ltd.) in which the electric buses are partlyreplaced by optical fibers, and both ends of the optical fibers areconnected to the electric buses via photo-electric conversion devices.

In a system using the controller S10/2 αand the device LWZ440, themaster of the control LAN is limited to only one controller and, hence,a signal that flows into the control LAN is either from master to slaveor from slave to master. Therefore, the optical linking device LWZ440changes over the direction of transmission in a unit of a packettransfer to realize the transmission of data between the master and theslave.

A conventional control system, in which the electric buses are partlyreplaced by optical fibers, is constituted by a master that outputs aninstruction to the control LAN and a plurality of slaves that operateupon receiving the instruction. This is because, when there exist manymasters, the control LAN itself must have an arbitration function tosimultaneously output control data (instructions) to the control LAN,and it becomes difficult to exchange the data in a predetermined periodin real time.

However, when the master is a controller, even a manual operation cannotbe accomplished from the operation board in case the controller becomesdefective. Therefore, an instruction system had to be separatelyprovided to halt the whole system in case of emergency.

To solve this problem, a multi-master system is required enabling aplurality of nodes connected to the control LAN to become masters. AnISO11898 standard is one of the transfer systems that corresponds to themulti-master system.

According to the transfer system of the ISO11898 standard as disclosedin Japanese Patent Laid-Open No. 236333/1994, a plurality of nodes areconnected using serial lines of the form of buses, enabling the data tobe simultaneously outputted to the LAN from a plurality of nodes.

According to this standard, furthermore, the data are transferred asevery node outputs data to the serial line and detects the state of thebus repetitively for every bit. Moreover, each node drives the bus atthe time when a logic 0 is outputted to the serial line but does notdrive the bus when a logic 1 is outputted, in order to transfer the databit by bit. Thus, even with one node, the bus assumes the state of logic0 when the logic 0 is outputted.

Therefore, every node detects the state of the bus after the data isoutputted. At this moment, the value outputted to the bus is comparedwith the state of the bus and when they are not in agreement, the nodeno more outputs the data. Thus, the nodes successively interrupt thetransmission of packet, thereby executing the arbitration.

In a system based on the ISO11898 standard, unlike the conventionalsystem of a single master, the states of all buses must be brought intoagreement while a bit is being transferred. In a system which changesover the direction of transmission using optical fibers in a unit of apacket as in the above-mentioned optical linking device (LWZ440),therefore, it is not allowed to bring the states of electric buses atboth ends of the optical fiber into agreement.

It is therefore presumed that the state of one electric bus istransmitted to the driven state, a logic 0 state, the driven stateoutputted by a logic 0 of another electric bus via an optical fiber.Optical bus-bridging devices attached to both ends of the optical fiberobserve the states of the electric buses to which they are connected,produce an optical output upon confirming that the electric bus is beingdriven, and transmit it to the other optical bus-bridging device via theoptical fiber. Upon detecting an optical input from the optical fiber,the other bus-bridging device drives the electric bus. Thus, the drivestate of the one electric bus is transmitted to the other electric busvia the optical fiber.

However, when the transmission of the state of the bus and the responseare executed in two directions in the optical linking devices by usingtwo optical fibers to realize a multi-master system, there may often beformed an optical loop by the two optical linking devices and theoptical fibers, resulting in the occurrence of a “deadlocked situation”or a “crossing situation” as described below, making it difficult toproperly bring the states into agreement.

FIG. 17 illustrates a problem stemming from the optical linking devicesof two directions. In this system, a node 1 and a node 2 drive theelectric buses a and b to which they are connected. Optical linkingdevices a and b are connected to both ends of the optical fibers, theoptical linking device a being connected to the bus a and the opticallinking device b being connected to the bus b. The optical linkingdevices a and b output light to the optical fibers when the electricbuses to which they are connected are driven. Conversely, when light isinputted from the optical fibers, the optical linking devices a and bdrive the electric buses to which they are connected.

(1) It is now presumed that none of the two electric buses a and b havebeen driven in the initial state. In this case, none of the buses a andb are driven, and none of the optical linking devices a and b areproducing optical output to the optical fiber, maintaining a stablestate.

(2) In this state, the node 1 connected to the bus a drives the bus a.

(3) Upon detecting the fact that the bus a is driven, the opticallinking device a produces an optical output to the optical fiber. Uponreceiving this optical output, the optical linking device b startsdriving the bus b.

(4) The bus b is driven by the optical linking device b, and the node 2detects the fact that the bus b is being driven. Since the bus b is in astate in which it is being driven, the optical linking device b producesoptical output to the optical fiber. Accordingly, the optical linkingdevice a starts driving the bus a.

(5) Next, the node 1 no longer drives the bus a. However, since theoptical linking device a continues to drive the bus a, the bus a ismaintained driven. Both the bus a and the bus b remain stable in a stateof being driven. Thus, a large latch loop is formed by the two opticalfibers and two optical linking devices. Finally, therefore, the buses aand b remain stable in a state of being driven despite they are drivenby none of the nodes, resulting in the occurrence of a so-called“deadlocked situation”.

Moreover, when the two electric buses a and b are driven to assume theON state during a transfer cycle, the optical linking devices a and b,respectively, judge that the buses of their own sides are turned ON andwork to produce optical outputs to the optical fibers in an effort toturn the buses of the other sides ON, establishing a “crossingsituation”. In the “crossing situation”, the bus drive signals of theoptical linking devices a and b are exchanged between the two buses;i.e., the buses a and b vibrate in repeating ON/OFF state.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an opticalcommunication method, optical linking devices and an opticalcommunication system which are free from the above-mentioned problemsinherent in the prior art, and are capable of bringing into properagreement the driven states of the two electric buses connected togetherthrough optical fibers, and in which a plurality of nodes are allowed tosimultaneously drive the buses.

The present invention is further concerned with a bus system in whichtwo electric buses are linked together through optical fibers, wherein amode of producing an optical output to the optical fiber is separatedfrom a mode of producing an electric output to the electric bus, inorder to prevent the formation of the above-mentioned optical loop.

The above-mentioned object is accomplished by an optical communicationmethod in which the states of the two electric buses connected throughoptical fibers are brought into agreement, wherein:

the states of said electric buses and the states of said optical fibersare observed while said electric buses are not being driven (OFF state);

while one or both of said electric buses are being driven (ON state) bythe nodes connected thereto, an optical output is continuously producedfrom the buses that are being driven to said optical fibers;

while light has been inputted from said optical fibers, the states ofsaid buses are not observed, but an electric output is produced to theelectric bus of the side to which light is inputted to drive the bus;and

when the buses are no longer driven by said nodes, said optical outputsand said electric outputs are halted, and said electric buses are nolonger driven.

This makes it possible to reliably avoid the above-mentioned deadlockedsituation.

Furthermore, at the time of being shifted to the non-driven mode by nolonger producing the electric output, a state is passed through in whichsaid electric buses are not observed for only a predetermined period oftime. Therefore, even when the optical bus-bridging devices are nolonger driving the electric buses, a transient ON state is noterroneously regarded the bus as driven despite the state of the electricbus has transiently changed from ON state to OFF state, and erroneousoperation is avoided. The above-mentioned predetermined period is longerthan a transient period. This transient period is determined by thecharacteristics of the means for driving the buses, and can be setirrespective of the transfer distance inclusive of optical fibers andelectric buses or the data transfer rate.

When both of said electric buses are driven by the respective nodes andwhen optical outputs are sent to said optical fibers from both sides,one side discontinues the production of said optical output and producessaid electric output only. This eliminates the above-mentioned “crossingsituation”.

The invention further deals with optical linking devices (opticalbus-bridging devices) for realizing the optical communication method ofthe present invention, installed among the optical fibers for connectingthe two electric buses and said electric buses in order to bring thestates of the two electric buses into agreement, comprising a meanswhich executes a standby mode for observing the states of the buses andthe states of the optical fibers when said electric buses are not beingdriven (OFF state), an optical output mode shifted from said standbymode when said electric buses are driven (ON state) by the nodes towhich they are connected, in order to produce an optical output to saidoptical fibers, a bus drive mode for producing an electric output to theelectric bus of its own side when an optical input is received from saidoptical fibers, and a non-observation mode which, when the buses are nolonger driven by said nodes, inhibits the observation of the states ofthe buses for a predetermined period of time at the time when said busdrive mode is shifted to said standby mode, wherein said means changesover these modes depending upon the states of the buses.

Provision is further made of a mode shift signal-setting means forshifting one of the two optical bus-bridging devices into the bus drivemode when the two electric buses are driven by the nodes and when thetwo optical bus-bridging devices provided on both sides of said opticalfibers are simultaneously changed over to said optical output mode.

The invention is further concerned with an optical communication systemto which the optical linking devices of the invention are adapted,comprising electric buses having two electrical states, a plurality ofnodes for outputting two-value data to said electric buses, opticallinking devices having means for converting electric signals intooptical signals and means for converting optical signals into electricsignals, and an optical fiber for connecting said two electric busestogether via said optical linking devices, wherein:

said optical fiber includes two optical fibers through which saidoptical linking devices execute optical output and optical inputseparately in order to transmit the states of said electric buses in twodirections;

said optical linking devices have a function for observing the ON/OFFstate of said electric buses and the presence/absence of optical inputfrom said optical fibers, for producing optical outputs to said opticalfibers when said electric bus is because optical linking devices areconnected only one electric bus in the ON state, and for producing anelectric output to the electric bus of its own side when an opticalinput is received from said optical fibers, and a function for haltingthe optical output of one side and for producing said electric outputonly when said two optical fibers have simultaneously produced saidoptical outputs giving rise to the occurrence of an optical loopsituation; and

when said electric buses are driven for each of the transmission cyclesdepending upon the ON/OFF of a bit data from said node, the drivenstates of the two electric buses are brought into agreement via saidoptical fibers and said optical linking devices on both sides thereof,and after the states have been brought into agreement, said nodesexecute the sampling of said electric buses.

When said data are simultaneously outputted from said plurality of nodesto said electric buses, the states of said buses are necessarilydetermined to be a preferential state, and every node compares the statein which it has produced an output to said electric bus with the stateof said electric bus and determines whether the data be continuouslyoutputted to said electric bus or not.

FIG. 18 illustrates the steps for bringing the driven states of theelectric buses into agreement according to the present invention.

(1) It is first presumed that none of the two electric buses a and bhave been driven in the initial state. In this case, the opticalbus-bridging devices a and b are both in the standby mode.

(2) Next, a node 1 connected to the bus a drives the bus a.

(3) The optical bus-bridging device a detects the fact that the bus a isdriven, shifts the mode from the standby mode into the optical outputmode, and produces an optical output to the optical fiber. Uponreceiving the optical output, the optical bus-bridging device b shiftsthe mode from the standby mode to the bus drive mode and starts drivingthe bus b. In the bus drive mode, no bus is observed.

(4) As a result, the bus b is driven, and the states of the two buses aand b are brought into agreement. The node 2 fetches the driven state ofthe bus b, and the transmission of a bit from the node 1 to the node 2ends.

(5) Next, the node 1 no longer drives the bus a. Thus, the opticalbus-bridging device a is shifted from the optical output mode to thestandby mode and no longer produces optical output to the opticalfibers. In response to this, the optical bus-bridging device b isshifted from the bus drive mode to the standby mode.

Thus, one of the optical bus-bridging devices is shifted to the opticaloutput mode and the other one is shifted to the bus drive mode to avoidthe occurrence of the deadlocked situation caused by the formation of anoptical loop.

In (5) above, a non-observation mode of a predetermined period of timeis passed through when the optical bus-bridging device b is shifted fromthe bus drive mode to the standby mode. This prevents the opticalbus-bridging devices from being erroneously operated.

When the two optical bus-bridging devices simultaneously assume theoptical output mode, furthermore, the device of the side in which a modetransition signal (MODE) has been set in advance changes the opticaloutput mode over to the bus drive mode.

In practice, a bus is driven by a node in compliance with the ISO11898standard for every period for transferring a bit. Therefore, the twooptical bus-bridging devices need not simultaneously share the opticaloutput mode but only one of them may have the optical output mode.

In order to accomplish the above-mentioned object, furthermore, thepresent invention deals with a data processing system comprising:

a first bus transferring voltage;

a second bus transferring voltage;

a plurality of computers connected to said first bus or said second bus,detect the state of the bus to determine whether the transmission can beeffected or not, and transmit and receive messages; and

a first bus-bridging device connected between said first bus and a thirdbus in order to connect said first bus to said second bus through saidthe third bus using light, and a second bus-bridging device connectedbetween said second bus and said third bus;

wherein said first and second bus-bridging devices have a function fordetecting whether a signal input from the third bus is the signal outputfrom the first and the second bus-bridging devices themselves or not.

In order to accomplish the above-mentioned object, furthermore, thepresent invention comprises:

a bus driver circuit connected to a first bus that uses transfersvoltage and exchanges the signals relative to said first bus;

a conversion circuit connected to a second bus using transferring lightand outputs optical signals to said first bus; and

a state-of-the-bus judging circuit which receives a signal representingthe state of said first bus sent from said bus driver circuit and asignal representing the state of said second bus sent from saidconversion circuit, determines the state of said first bus based upon achange in the signal representing the state of said first bus and upon achange in the state of said second bus, and sends an output to said busdriver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the constitution of an opticalcommunication system according to an embodiment of the presentinvention;

FIG. 2 is a diagram illustrating the constitution of an opticalbus-bridging device;

FIG. 3 is a diagram illustrating the constitution of a bus drivercircuit;

FIG. 4 is a diagram illustrating the operation specifications of atransmitter and a receiver, and a packet structure (ISO11898 standard);

FIG. 5 is a diagram illustrating the constitution of a photo-electricconverter;

FIG. 6 is a diagram illustrating the operation specifications of aphoto-electro converter and an electro-photo converter;

FIG. 7 is a diagram illustrating the constitution of an operation modeshifting device;

FIG. 8 is a diagram illustrating the constitution of a synchronizingcircuit;

FIG. 9 is a diagram illustrating the constitution of a timer device;

FIG. 10 is a diagram illustrating the operation of a decrementer;

FIG. 11 is a diagram illustrating the constitution of acondition-setting device;

FIG. 12 is a diagram illustrating the mode-shifting operation of a modechange-over circuit;

FIG. 13 is a diagram illustrating another mode-shifting operation of themode change-over circuit;

FIG. 14 is a diagram of a timing chart illustrating the operation of theoptical communication system;

FIG. 15 is a diagram of a timing chart illustrating another operation ofthe optical communication system;

FIG. 16 is a diagram illustrating the constitution of a water supply anddrainage system to which the present invention is applied;

FIG. 17 is a diagram illustrating a problem at the time of connectingoptical fibers between the electric buses;

FIG. 18 is a diagram schematically illustrating the steps for bringinginto agreement the buses using the optical bus-bridging devices of thepresent invention;

FIG. 19 is a diagram illustrating the operations for arbitrating thesimultaneous packet transmission from a plurality of nodes and forbringing the states of the buses into agreement;

FIG. 20 is a diagram illustrating the constitution of another opticalbus-bridging device;

FIG. 21 is a diagram illustrating the constitution of a state-of-the-busjudging circuit;

FIG. 22 is a diagram illustrating the constitution of a drivingcondition judging circuit;

FIG. 23 is a diagram illustrating the constitution of a synchronizingcircuit;

FIG. 24 is a diagram illustrating the constitution of a drivesignal-generating circuit;

FIG. 25 is a diagram illustrating the operation of a set conditioncircuit;

FIG. 26 is a diagram illustrating the operation of a time countercircuit;

FIG. 27 is a diagram of a time chart illustrating the operation of thetime counter circuit;

FIG. 28 is a diagram illustrating the constitution of a time-settingcircuit;

FIG. 29 is a diagram illustrating the state transition graph by thestate-of-the-bus judging circuit;

FIG. 30 is a time chart illustrating the operation of the bus-bridgingdevice; and

FIG. 31 is a time chart illustrating the operation of the bus-bridgingdevice.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detailwith reference to the drawings.

FIG. 1 illustrates the constitution of an optical communication systemaccording to an embodiment of the present invention. Two electric busesA and B to which nodes 30 are connected are connected together throughoptical bus-bridging devices 10 a, 10 b and an optical fiber 50, therebyto constitute a network. As will be described later, the opticalbus-bridging device 10 constitutes a nucleus portion of the presentinvention.

A bus system A 1 a is constituted by the optical bus-bridging device 10a and nodes 30 a, 30 b that are electrically connected to an electricbus A 20 a (hereinafter referred to as bus 20 a). Similarly, a bussystem B 1 b is constituted by the optical bus-bridging device 10 b andnodes 30 c, 30 d that are electrically connected to an electric bus B 20b (hereinafter referred to as bus 20 b). The optical fiber 50 isconstituted by two optical fibers, i.e., an optical fiber whichtransmits an optical output of the optical bus-bridging device 10 a tothe optical bus-bridging device 10 b and an optical fiber whichtransmits an optical output of the optical bus-bridging device 10 b tothe optical bus-bridging device 10 a.

Every node 30 repeats the transfer of data of a bit to the bus and thedetection of the state of the bus, which is called transfer cycle,according to the protocol specified under the ISO11898. Therefore, theoutput of a given node 30 must have been fed to every node 30 prior tothe sampling point SP for detecting the state of the bus at the end ofthe transfer cycle.

In order to realize this, the optical bus-bridging device 10 suitablychanges over a standby mode for detecting the state of the bus, anoptical output mode for transferring the driven state of the bus of itsown side to the opposing side, a bus drive mode for driving the bus ofits own side depending upon the optical input, and a non-observationmode for inhibiting the observation of the state of the bus for apredetermined period of time when the bus drive mode is shifted to thestandby mode, during the transfer cycle in order to bring the states ofthe buses 10 a and 10 b into proper agreement.

Described below is the operation of the system starting from the standbymode in which neither the bus 20 a nor the bus 20 b is driven throughthe situation in which the bus 20 a is driven, the situation in whichthe bus 20 a is no longer driven in the next transfer cycle up to thestandby mode of the next time.

As the node 30 a or the node 30 b drives the bus 20 a, the opticalbus-bridging device 10 a is changed over to the optical output mode fortransmitting the driven state of the bus 20 a to the system B of theopposing side, and sends an optical output which is a request for driveto the optical fiber 50. Upon receipt of the request for drive from thesystem A through the optical fiber 50, on the other hand, the opticalbus-bridging device 10 b is changed over to the bus drive mode fortransmitting the state of the bus 20 a to the bus of its own side, andstarts driving the bus 20 b. As a result, the bus 20 a and the bus 20 bare put in the state in which they are driven in agreement with eachother. Furthermore, the nodes 30 c and 30 d detect the fact that the bus20 b is in the state of being driven. That is, the data (ON) of a bit istransferred from the system A to the system B.

When the bus 20 a is no longer driven in the next transfer cycle, theoptical bus-bridging device 10 a is changed over to the standby mode,and no optical output is sent to the optical fiber 50. Since there is norequest for driving the bus from the system A, the optical bus-bridgingdevice 10 b no longer drives the bus 20 b. At this moment, if theoptical bus-bridging device 10 b is readily changed over to the standbymode from the bus drive mode to observe the bus 20 b which is still inan electrically transient period after being driven by the opticalbus-bridging device 10 b, then, the optical bus-bridging device 10 b mayerroneously judge that the bus 20 b is being driven and erroneouslyproduces an optical output.

Prior to being shifted to the standby mode, therefore, the opticalbus-bridging device 10 b is shifted to the non-observation mode in whichit is inhibited to observe the bus 20 b for a predetermined period oftime. After the passage of a predetermined period of time, the bus 20 bstably assumes the OFF state from the ON state. Then, the opticalbus-bridging device 10 b is shifted to the standby mode to observe thebus 20 b again. As a result, neither the bus 20 a nor the bus 20 b isdriven.

When the observation is resumed after the passage of a predeterminedperiod of time to detect that the bus 20 b is in a state of beingdriven, it is then confirmed that the node 30 c or the node 30 dconnected to the bus 20 b has worked. Then, the optical bus-bridgingdevice 10 b is shifted to the optical output mode to start sendingoptical output to the optical fiber 50. Upon receipt of the opticaloutput, the optical bus-bridging device 10 a drives the bus 20 a.

FIG. 2 illustrates the constitution of the optical bus-bridging device.The optical bus-bridging device 10 has the same constitution in eitherthe system A or the system B and is, hereinafter, described withoutdistinction of the system. As required, furthermore, a sign of thesignal line is attached in parenthesis to the end of the name of thesignal.

The optical bus-bridging device 10 is constituted by a bus drivercircuit 11 for driving and detecting the state of the bus 20, aphoto-electric converter 13 which converts an electric signal into lightto output it into the optical fiber 50 and converts an optical inputfrom the optical fiber 50 into an electric signal, and an operation modeshifting device 12.

The bus driver circuit 11 and the operation mode shifting device 12 areconnected together through a signal line 131 of a signal RxD_N and asignal line 134 of a signal TxD_N, and the operation mode shiftingdevice 12 and the optical converter 13 are connected together through asignal line 132 of DOUT and a signal line 133 of DIN.

FIG. 3 is a diagram schematically illustrating the bus driver circuit.The bus driver circuit 11 is constituted by a receiver 1101 fordetecting the state of the bus 20 and a transmitter 1102 for driving thebus 20. The bus 20 comprises two signal lines 31 and 32 for transmittingsignals BUSH and BUSL. The signals BUSH (31) and BUSL (32) are outputsof the transmitter 1102 and inputs to the receiver 1101.

FIG. 4 illustrates the operations of the transmitter and the receiver,and the constitution of a packet. FIG. 4(a) illustrates the operation ofthe transmitter 1102. When a value of the input signal TxD_N (134) is ON(0), the signals BUSH (31) and BUSL (32) change into predeterminedvoltages Vonh and Vonl, and the bus 20 assumes the ON state. When thevalue of TxD_N is OFF (1), on the other hand, the transmitter 1102 doesnot drive the bus 20, the signals BUSH and BUSL both assume apredetermined voltage Voff, and the bus 20 assumes the OFF state.

FIG. 4(b) illustrates the operation of the receiver 1101. When apotential difference Vdf between BUS H (31) and BUSL (32) is greaterthan a threshold value Vth, the receiver 1101 judges the bus 20 to be inthe ON state, and turns the signal RxD_N of the signal line 131 ON (0).When the potential difference Vdf is smaller than the threshold valueVth, on the other hand, the receiver 1101 judges the bus 20 to be in theOFF state and turns the signal RxD_N (131) OFF (1).

In the bus driver circuit 11, the inputs of the transmitter 1102 and thereceiver 1101 are connected together via BUSH (31) and BUSL (32).Therefore, when the input TxD_N (134) of the transmitter 1102 is turnedON, the potential difference Vdf between BUSH (31) and BUSL (32) exceedsthe threshold value Vth, and the output RxD_N (131) of the receiver 1101is turned ON. The time until the bus 20 assumes the OFF state after thebus 20 is no longer driven to remain in its ON state by the bus drivercircuit 11, is determined by the characteristics of the bus drivercircuit 11. Therefore, the period of the non-observation mode isdetermined by the characteristics of the bus driver circuit 11 onlyirrespective of the transfer distance (lengths of optical fiber 50 andbus 20) or the data transfer rate.

It is now presumed that Voff=2.5 (v) and Vth=0.8 (v) when Vonh=3.5 (v)and Vonl=1.5 (v). When the transmitter 1102 drives the bus to assume theON state, the potential difference Vdf between BUSH and BUSL isVonh−Vonl =2.0 (v). Since Vdf≧Vth, the receiver 1101 detects the ONstate of the bus 20. When the transmitter 1102 does not work and the bus20 is in the OFF state, the potential difference Vdf between BUSH andBUSL is 0 (v) and Vdf<Vth. Therefore, the receiver 1101 detects the OFFstate of the bus 20.

The plurality of nodes 30 connected to the bus 20 are equipped with abus driver circuit (not shown) same as the bus driver circuit 11. Thebus driver circuits in the nodes 30 are capable of driving the bus 20simultaneously. When there is at least one node 30 capable of drivingthe bus 20 to assume the ON state, there develops a potential differencebetween BUSH (31) and BUSL (32), and the bus assumes the ON state. Inthis case, other nodes 30 that did not drive the bus 20 in an attempt tomaintain the bus 20 in the OFF state, detect the ON state of the bus 20.

In view of this feature, the packet stipulated under the ISO11898standard has been constituted by a header 7001, a data body 7002 and atail 7003 as shown in FIG. 4 (c). The node 30 compares its own outputwith the state of the bus for every bit while the header 7001 is beingtransferred. When the states are not in agreement, the node 30 executesthe arbitration processing to no longer transfer the packet. Therefore,in a step of entering into the transfer of the data body 7002, only onenode is transferring data to the bus.

FIG. 5 is a diagram schematically illustrating the photo-electricconverter. The photo-electric converter 13 is constituted by aphoto-electric conversion unit O/E 1301 for converting light intoelectricity and an electric-photo conversion unit E/O 1302 forconverting electricity into light. In FIG. 5, DIN (133) denotes anelectric control signal for turning an optical output ON and OFF, andDOUT (132) denotes a control signal for turning the input TxD_N (134) ofthe transmitter 1102 ON and OFF.

FIG. 6 is a diagram illustrating the operation of the photo-electricconverter. The photo-electric conversion unit O/E 1301 operates asillustrated in FIG. 6(a). That is, the electric output DOUT (132) isturned OFF (0) when there is no optical input from the optical fiber 50,i.e., when OIN (1311) is OFF, and is turned ON (1) when OIN (1311) isON.

The electric-photo conversion unit E/O 1302 operates as shown in FIG.6(b). That is, when the electric input DIN (133) is OFF (0), the opticaloutput OOUT (1312) is turned OFF. When DIN (133) is ON (1), OOUT (1312)is turned ON (1) and an optical output is sent to the optical fiber 50.

FIG. 7 is a diagram schematically illustrating an operation modeshifting device. The operation mode shifting device 12 is constituted byan operation mode shifting circuit 121 and a condition setting device122. The operation mode shifting circuit 121 is constituted by a modechange-over circuit 200, a synchronizing circuit 300 and a timer device400.

The synchronizing circuit 300 receives the output RxD_N (131) of thereceiver 1101 in the bus driver circuit 11 and the output DOUT (132) ofthe photo-electric conversion unit 1301, turns the output RxD_N intoSRxD (141) and turns DOUT into SDOUT (142) in synchronism with a clockof the mode change-over circuit 200, and sends them to the modechange-over circuit 200.

The mode change-over circuit 200 receives SRxD (141), SDOUT (142), atime-up signal CNT_UP (143) from the timer device 400 and a modeshift-setting signal MODE (151) from the condition setting device 122,and produces an input DIN (133) to the electric-photo conversion unit1302, an input TxD_N (134) to the transmitter 1102, and a request fortimer operation CNT_ENB (144) to the timer device 400.

Upon receipt of the request for timer operation CNT_ENB, the timerdevice 400 outputs the time-up signal CNT_UP (143) to the modechange-over circuit 200 after having counted the timer count number CNT(152) set by the condition setting device 122.

FIG. 8 illustrates the constitution of a synchronizing circuit. Thesynchronizing circuit 300 is constituted by a NOT circuit 3011, and fourlatches 3001 to 3004 for fetching data in synchronism with the clocks ofthe mode change-over circuit 200.

The signals RxD_N (131) and DOUT (132) inputted to the synchronizingcircuit 300 change out of synchronism with the clocks of the modechange-over circuit 200. Therefore, the signals RxD_N and DOUT areconverted, through two stages of latches, i.e., latches 3001, 3002 andlatches 3003, 3004, into signals SRxD (141) and SDOUT (142) insynchronism with the clocks of the mode change-over circuit 200. Thesignal RxD_N is a negative logic signal and is inputted to the latch3001 after it is converted into a positive logic signal through the NOTcircuit 3011.

FIG. 9 illustrates the constitution of a timer device. The timer device400 is constituted by a selector 4012, a 4-bit decrementer 4001 and4-bit-width latches 4011. The selector 4012 selects the output of thelatch 4011 when the request for timer operation CNT_ENB (144) is ON (1),and selects the timer count number (152) when the CNT_ENB is OFF (0) andsends it to the 4-bit decrementer 4001. The 4-bit decrementer 4001 sendsa value obtained by subtracting 1 from the output (4021) of the selector4012 as an output (4022) to the latch 4011. When the input (4021) to the4-bit decrementer 4001 is 0, a value 1 is outputted to CNT_UP (143).When the input (4021) is not 0, a value 0 is output to CNT_UP (143).

FIG. 10 is a truth table of the 4-bit decrementer. When the request fortimer operation CNT_ENB (144) from the mode change-over circuit 200 isOFF, the selector 4021 in the timer device 400 selects the timer countnumber, and the latch 4011 latches a value obtained by subtracting 1from the timer count number CNT. Next, as the CNT_ENB (144) is turnedON, the selector 4012 selects the output of the latch 4011. Therefore,the value held by the latch 4011 decreases by 1 every time when theclock rises. When the value thus held becomes 0, i.e., when the output(4021) of the latch 4011 becomes 0, a value 1 is outputted to the CNT_UP(143).

Thus, when the clocks are counted by the amount of the timer countnumber CNT after the request for timer operation CNT_ENB is turned ON,the timer device 400 outputs a time-up signal CNT_UP. The period ofcounting the count number CNT is a period of the non-observation modethat will be described later. As described earlier, the period of thenon-observation mode is determined by the characteristics of the busdriver circuit 11 only irrespective of the transfer distance or thetransfer rate.

FIG. 11 is a diagram schematically illustrating a condition settingdevice. The condition setting device 122 is constituted by a 5-bitsetting switch 1221 and a pull-up resistor 1222. A signal 0 is outputtedto a signal line having a closed contact, and a signal 1 is outputted toan open signal line for every bit. Among the outputs of five bits, a bitof a signal line 151 forms a mode shift-setting signal MODE, and fourbits of four signal lines 152 form a timer count number CNT. The modeshift-setting signals MODEs of the two optical bus-bridging devicesconnected through an optical fiber are so set as will be opposite toeach other. As will be described later, the optical bus-bridging device10 in which MODE=0 is set, shifts its own mode to the bus drive modewhen it conflicts with other optical output modes.

Next, described below is the operation mode shifting circuit 121. Inorder that the states of the buses 20 a and 20 b connected togetherthrough the optical fiber 50 are brought into proper agreement in bothdirections, the operation mode shifting circuit 121 judges the state ofthe bus of its own side and the state of optical input from the opticalfiber, and the portions of the optical bus-bridging device 10 executethe corresponding operations depending upon the mode changed over by themode change-over circuit 200.

FIGS. 12 and 13 are diagrams illustrating the operation for changingover the mode of the operation mode shifting device. Both FIGS. 12(a)and 13(a) are diagrams illustrating the shift of the states uponchanging over the mode, and illustrate a standby mode Q0 in whichneither optical output nor bus drive is effected, a bus drive mode Q1 inwhich the bus only is driven, an optical output mode Q2 in which anoptical output only is effected, and a non-observation mode Q3 which ispassed through when the bus drive mode Q1 is shifted to the standby modeQ0.

In FIG. 12, the optical output mode Q2 is not shifted to the bus drivemode Q1 under the condition where the mode shift-setting signal MODE(151)=1. FIG. 12(b) shows the outputs of the mode change-over circuit200.

First, described below is the operation in the standby mode Q0. In themode Q0, the bus 20 which is the input to the optical bus-bridgingdevice 10 is in the OFF state and the optical input OIN (1311) to thephoto-electric converter 13 is OFF. Therefore, the outputs SRxD (141)and SDOUT (142) of the synchronizing circuit 300 both assume the logic0. Accordingly, the mode change-over circuit 200 produces DIN (133)which is OFF, TxD_N (134) which is OFF and CNT_ENB (144) which is OFF.Therefore, the optical output OOUT (1312) is not sent to the opticalfiber 50, the bus 20 is not driven (BUSH (31)−BUSL (32)<Vth), and thetimer device 400 does not work.

When the bus 20 is driven to assume the ON state during the standby modeQ0, the output RxD_N (131) of the bus driver circuit 11 is turned ON,and the output SRxD (141) of the synchronizing circuit 300 changes fromthe logic 0 to the logic 1. In response to this, the mode change-overcircuit 200 shifts the mode from the mode Q0 into the optical outputmode Q2.

When the light is inputted to the photo-electric converter 13 throughthe optical fiber 50 during the standby mode Q0, furthermore, DOUT (132)is turned ON. Accordingly, the output SDOUT (142) of the synchronizingcircuit 300 changes from the logic 0 to the logic 1. In response tothis, the mode change-over circuit 200 shifts the mode from the mode Q0into the bus drive mode Q1.

Thus, when the drive mode of the bus 20 of its own side is observed, theoperation mode shifting device 12 in which MODE=1 has been set, changesthe mode from the standby mode Q0 into the optical output mode Q2 butdoes not shift the mode from the mode Q2 to the mode Q1. When theoptical input from the optical fiber 50 is observed, on the other hand,the mode is shifted from the mode Q0 to the mode Q1. This makes itpossible to avoid the formation of the above-mentioned optical loop.

Next, described below is the operation in the bus drive mode Q1. In themode Q1, the mode change-over circuit 200 outputs DIN (133) which isOFF, TxD_N (134) which is ON and CNT_ENB (144) which is OFF. Therefore,the transmitter 1102 in the bus driver circuit 11 is turned ON, and thebus 20 a is driven. On the other hand, since DIN (133) is OFF, no outputis sent to the optical fiber 50, and the timer device 400 does notoperate.

When there is an optical input from the optical fiber 50 (i.e., whenthere is a request for drive from the other system) in the mode Q1, theoutput DOUT (132) of the photo-electric converter 13 is turned ON, theoutput SDOUT (142) of the synchronizing circuit 300 maintains the logic1, and the bus 20 is maintained in the driven mode.

Due to the structure of the bus driver circuit 11, furthermore, theoutput of the transmitter 1102 directly serves as an input to thereceiver 1101. Therefore, when the TxD_N (134) is turned ON and thetransmitter 1102 is turned ON to drive the bus 20 a, the receiver 1101is turned ON and RxD_N (131) is turned ON. Accordingly, the output SRxD(141) of the synchronizing circuit 300 assumes the logic 1. However,since the mode change-over circuit 200 does not observe the output SRxD(141) in the bus drive mode Q1, the operation is not affected by thechange of the synchronizing circuit 300.

When the optical input from the optical fiber 50 extinguishes in the busdrive mode Q1, the output DOUT (132) of the photo-electric converter 13is turned OFF, and the output SDOUT (142) of the synchronizing circuit300 assumes the logic 0. In response to this, the mode change-overcircuit 200 changes the mode from the mode Q1 to the non-observationmode Q3.

In the bus drive mode Q1 as described above, the ON mode is continuedfor a short period of time after TxD_N (134) is turned OFF. Thistransient period is the non-observation mode which prevents the opticalbus-bridging device 10 from erroneously judging that the bus is beingdriven.

Next, described below is the operation in the optical output mode Q2. Inthe mode Q2, the output DIN (133) of the mode change-over circuit 200 isturned ON, TxD_N (134) is turned OFF and CNT_ENB (144) is turned OFF.Therefore, OOUT (1312) of the photoelectric converter 13 is turned ONand an optical output is sent onto the optical fiber 50. On the otherhand, since TxD_N (134) is turned OFF, the bus 20 is not driven and thetimer device 400 does not operate.

While the bus 20 has been driven in the optical output mode Q2, theoutput RxD_N (131) of the receiver 1101 in the bus driver circuit 11 isturned ON. Accordingly, SRxD (141) maintains the logic 1 and the modechange-over circuit 200 maintains the mode Q2.

When there is an optical input from the other system through the opticalfiber 50 while the optical bus-bridging device 10 is in the opticaloutput mode Q2, DOUT (132) is turned ON and SDOUT (142) is turned ON,resulting in the occurrence of the above-mentioned “crossing situation”.When MODE (151)=1 is being set by the condition setting device 122,however, the mode change-over circuit 200 does not observe SDOUT (142)in the mode Q2. Therefore, the “crossing situation” does not occur, andthe optical output mode Q2 is stably maintained.

When the bus 20 assumes the OFF mode in the optical output mode Q2, theoutput RxD_N (131) of the bus driver circuit 11 is turned OFF, and theoutput SRxD (141) of the synchronizing circuit 300 changes from thelogic 1 to the logic 0. In response to this, the mode change-overcircuit 200 shifts the mode from the mode Q2 to the standby mode Q0.Unlike the case of the mode Q1, the mode Q2 is shifted to the mode Q0after the mode of the bus 20 has been stabilized. Therefore, the inputSRxD (141) to the mode change-over circuit 200 is not erroneouslyregarded to be turned on, and no error occurs in the operation despitethe mode is readily shifted to the standby mode Q0.

Next, described below is the operation in the non-observation mode Q3.In the mode Q3, the output DIN (133) of the mode change-over circuit 200is turned OFF, TxD_N (134) is turned OFF and CNT_ENB (144) is turned ON.Therefore, the timer 400 operates, effects the counting set by thecondition setting device for every clock, and outputs the time-up signalCNT_UP (143) when the preset count number CNT (152) is reached.

In the mode Q3, the mode change-over circuit 200 observes neither SRxD(141) nor SDOUT (142), and observes the output CNT_UP (143) of the timerdevice 400 only. The mode Q3 is maintained during the period in whichCNT_UP=0. In the non-observation period in the mode Q3, the opticalbus-bridging device 10 no longer drives the bus 20, and the bus 20stably assumes the OFF mode. When the output CNT_UP=1 is produced, themode change-over circuit 200 shifts the mode from the mode Q3 to themode Q0, and observes SRxD (141) and SDOUT (142) again. This preventsthe optical bus-bridging device 10 from being erroneously operated dueto a transient change in the mode of the bus.

In FIG. 13, the optical output mode Q2 is shifted to the bus drive modeQ1 under the condition where the mode shift-setting signal MODE (151)=0.The diagram of shifting the mode in FIG. 13(a) is different from that ofFIG. 12(a) only in regard to the operation under the “crossingsituation” in the optical output mode Q2. The difference will now bedescribed.

In the optical output mode Q2, MODE is 0 and, hence, the modechange-over circuit 200 observes the change in the output DOUT (132) ofthe photo-electric converter 13. When there is an optical input from theoptical fiber 50, therefore, DOUT (132) is turned ON and SDOUT (142)changes from the logic 0 to the logic 1. In response to this, the modechange-over circuit 200 judges that the “crossing situation” isoccurring and shifts the mode Q2 into the bus drive mode Q1. As aresult, DIN (133) is turned OFF and TxD_N (134) is turned ON, wherebythe optical output of the photo-electric converter 13 a is turned OFF,and the “crossing situation” is eliminated.

Accordingly, despite the buses 20 a and 20 b are driven to assume the ONmode during the same transfer cycle, the buses are not vibrated due tothe “crossing situation”; i.e., the buses 20 a and 20 b are brought intoproper agreement.

FIG. 14 is a timing chart illustrating the operation of an opticalcommunication system according to the embodiment. FIG. 14 illustratesthe operation of every portion of the system of FIG. 1 in a mode wherethe node 30 a (or 30 b) connected to the bus 20 a drives the bus 20 a toassume the ON mode in a given transfer cycle and no longer drives thebus 20 a in a next transfer cycle so that it assumes the OFF mode.

First, the optical bus-bridging devices 10 a and 10 b of the systems Aand B are both in the standby mode Q0. When the bus 20 a of the system Aassumes the ON mode, the output RxD_N (131) of the bus driver circuitlla changes from the logic 1 to the logic 0, and the input of the modechange-over circuit 200 (output of the synchronizing circuit 300) SRxD(141) changes from the logic 0 to the logic 1. In accordance with thediagram of shift of FIG. 12(a), the mode change-over circuit 200 shiftsthe mode from the standby mode Q0 to the optical output mode Q2, and theoutput DIN (133) changes from the logic 0 to the logic 1.

In response to a change in the output DIN (133), the optical output OOUT(1312) of the photo-electric converter 13 a is turned ON and istransmitted to the optical bus-bridging device 10 b of the system Bthrough the optical fiber 50. The optical input OIN (1311) of thephoto-electric converter 13 b is turned ON, and the output DOUT (132)changes from the logic 0 to the logic 1. A delay due to the opticaltransmission time occurs from a change in the DIN (133) in the system Aup to the DOUT (132) in the system B.

Upon receipt of DOUT (132), the mode change-over circuit 200 in thesystem B shifts the mode from the standby mode Q0 to the bus drive modeQ1 as shown in FIG. 12(a), and the output TxD_N (134) changes from thelogic 1 to the logic 0. Upon receipt of a change in the TxD_N (134), thebus driver circuit 11 b drives the bus B 20 b to assume the ON mode.

As described above, the ON mode of the bus 20 a is transferred to thebus 20 b through the optical bus-bridging devices 10 a and 10 b, and thenode 30 b (or 30 a) fetches the mode from the bus 20 a and the nodes 30c and 30 d fetch the mode from the bus B 20 b at the sampling point SP1at the end of the transfer time of one bit, to make sure that the bus isin the ON mode. That is, the data of one bit is transferred as the bus20 is driven by the node 30 a to assume the ON mode.

When the node 30 a (or 30 b) no longer drives the bus 20 a in the nexttransfer cycle, the output RxD_N (131) of the bus driver circuit 11 achanges from the logic 0 to the logic 1. Due to this change, the modechange-over circuit 200 shifts the mode from the mode Q2 to the mode Q0,whereby the output DIN (133) changes from the logic 0 to the logic 1,and the optical output OOUT (132) of the photo-electric converter 13 ais turned OFF.

In the optical bus-bridging device 20 b of the system B, therefore, theoptical input OIN (1311) from the optical fiber 50 is turned OFF, andthe output DOUT (132) changes from the logic 1 to the logic 0. In thiscase, too, a delay occurs due to the optical transmission time. As theoutput DOUT (132) changes to 0, the mode change-over circuit 200 shiftsthe mode from the mode Q1 to the mode Q3 in accordance with FIGS. 12(a).

In the non-observation mode Q3, the mode change-over circuit 200 changesCNT_ENB (144) from the logic 0 to the logic 1 to operate the timerdevice 400, and neglects the input SRxD (141) by the observed valueRxD_N (131). In the non-observation mode Q3, furthermore, the nodes 30 cand 30 d are entering into the next bit transfer cycle and, hence, thebus 20 b may have been driven by the nodes 30 c and 30 d to assume theON mode (in an example of FIG. 14, the bus 20 b has not been driven buthas been changed from the ON mode to the OFF mode).

In the non-observation mode Q3, therefore, a mode is imparted in whichthe bus 20 b is not observed in order to prevent the transient period inwhich the bus 20 b changes from the ON mode to the OFF mode from beingerroneously regarded to be in the mode that the bus is being driven by anode in the system B. When the nodes 30 c and 30 d are driving the bus20 b to assume the ON mode, SRxD (141) is observed again when thenon-observation mode Q3 is shifted to the standby mode Q0. Accordingly,the mode is shifted to the optical output mode Q2 to properly recognizethe mode of the bus 20 b.

When the output CNT_UP (143) of the timer device 400 changes from thelogic 0 to the logic 1, the mode change-over circuit 200 shifts the modefrom the mode Q3 to the mode Q0 to observe the bus 20 b again. When notbeing driven by the nodes 30 c, 30 d of the system B as in theembodiment of FIG. 14, therefore, the bus 20 b assumes the OFF mode, andthe nodes 30 b to 30 d fetch the OFF mode at a sampling point SP2.

FIG. 15 is a timing chart of a mode different from that of FIG. 14. Inthis example, the buses 20 a and 20 b are driven to assume the ON modein a given transfer cycle and in a next transfer cycle, MODE (151)=1 isset to the operation mode shifting device 12 a of the system A and MODE(151)=0 is set to the operation mode shifting device 12 b of the systemB in a mode where the bus 20 has not been driven.

When the buses 20 a and 20 b are driven to assume the ON mode in thesame transfer cycle, the input RxD_N (131) from the bus driver circuits11 a, 11 b changes from the logic 1 to the logic 0. In response to thischange, both the operation mode shifting devices 12 a and 12 b shift themode from the mode Q0 to the mode Q2, whereby the output DIN (133)changes from the logic 0 to the logic 1, and the photo-electricconverters 13 a and 13 b produce optical outputs OOUT (1312) which areturned ON.

Therefore, the ON mode is transmitted in the two directions through twooptical fibers 50, and the electric outputs DOUT (132) of thephoto-electric converters 13 a and 13 b change from the logic 0 to thelogic 1. In this case, since MODE (151)=0, the operation mode shiftingdevice 12 b changes the mode from the mode Q2 to the mode Q1 as shown inFIG. 13(a), whereby both the outputs DIN (133) and TxD_N (134) changefrom the logic 1 to the logic 0, the optical output OOUT (1312) of thephoto-electric converter 13 b is turned OFF, and the bus driver circuit11 b drives the bus 20 b to assume the ON mode.

At the sampling point SP1, the nodes 30 a to 30 d fetch the states ofthe buses 20 a and 20 b, and fetch a bit of the ON mode.

In the next transfer cycle, the node 30 a (or 30 b) no longer drives thebus 20 a which is in the ON mode. Therefore, the operation mode shiftingdevice 12 a shifts the mode from the mode Q2 to the mode Q0, and theoutput DIN (133) changes from the logic 1 to the logic 0. Accordingly,the optical output OOUT (1312) of the photo-electric converter 13 a isturned OFF.

Upon receipt of a change in the optical output OOUT (1312), the electricoutput DOUT (132) of the photo-electric converter 13 b changes from thelogic 1 to the logic 0, the operation mode shifting device 12 b shiftsthe mode from the mode Q1 to the mode Q3, changes CNT_ENB (144) from thelogic 0 to the logic 1, and causes the timer device 400 to operate.Then, when the output CNT_UP (143) of the timer device 400 changes fromthe logic 0 to the logic 1, the operation mode shifting device 12 bshifts the mode from the mode Q3 to the mode Q0. However, since the node30 c (or 30 d) is driving the bus 20 b, the input RxD_N (131) to theoperation mode shifting device 12 b still maintains the logic 0.

Accordingly, the operation mode shifting device 12 b shifts the modefrom the mode Q0 to the mode Q2, changes the output DIN (133) from thelogic 0 to the logic 1, and whereby the optical output OOUT (1312) ofthe photo-electric converter 13 b is turned ON. The photo-electricconverter 13 a that has received this change through the optical fiber50 changes the electric output DOUT (132) from the logic 0 to the logic1.

Due to a change in the output DOUT (132), the operation mode shiftingdevice 12 a shifts the mode from the mode Q0 to the mode Q1, changesTxD_N (134) from the logic 1 to the logic 0, and the bus driver circuit11 a drives the bus 20 a to assume the ON mode. At the sampling pointSP2, therefore, the nodes 30 a to 30 d fetch the ON mode from the buses20 a and 20 b.

As described above, the buses 20 a and 20 b are brought into agreement,and the data are properly transferred.

Next, described below is an example to which the present invention isadapted. FIG. 16 illustrates a water supply system to which the opticalbus-bridging devices of the present invention are applied. A controller8001 controls a pump 8012 through a motor controller 8002, and maintainsthe pressure constant in a main water supply tube 8110 to supply waterin proper amounts.

The controller 8001 is connected to the motor controller 8002 and to theoptical bus-bridging device 10 a through the electric bus 20 a. Anoperation board 8011 is connected to a sensor 8013 attached to the mainwater supply tube 8110 and to the optical bus-bridging device 10 bthrough the electric bus 20 b. The optical bus-bridging devices 10 a and10 b are connected together through the optical fiber 50. The motorcontroller 8002 and a motor 8102 for driving a pump 8012 are connectedtogether through a power line 8101. The sensor 8013 is a pressure gaugewhich indicates a pressure in the main water supply tube 8110 producedby the pump. The mode shift-setting signals of the optical bus-bridgingdevices 10 a and 10 b have been set to be opposite relative to eachother.

The controller 8001, the motor controller 8002 and the sensor 8013constituting the system, are equipped with known functions for effectingthe communication according to a communication protocol stipulated underISO11898; i.e., a multi-master system is constituted in which both thecontroller 8001 and the operation board 8011 can become masters.

Described below is the operation of the water supply system. Thecontroller 8001 requests the sensor 8013 to output the pressure in themain water supply tube 8110, and the sensor 8013 outputs the pressure inthe main water supply tube 8110. Next, the controller 8001 sends aninstruction to the motor controller 8002 to raise the pressure when thepressure in the main water supply tube 8110 is lower than apredetermined value and, on the other hand, to lower the pressure whenthe pressure is too high. Upon receipt of an instruction from thecontroller 8001, the motor controller 8002 controls the pump 8012 bychanging the current and voltage fed to the motor 8102 that drives thepump 8012, in order to change the pressure in the main water supply tube8110. The above-mentioned operation is executed every time after apredetermined period in order to maintain a predetermined pressure inthe main water supply tube 8110.

Next, described below is a processing from the operation board 8011. Incase the controller 8001 becomes defective or the pump 8012 must bestopped due to emergency, an operator manually operates the operationboard 8011 to send an instruction. The operation board 8011 includes amanual/automatic change-over switch, an emergency stop switch forstopping the pump 8012 in case of emergency, a dial for setting avoltage and a current supplied to the motor 8102, a meter for indicatingthe pressure in the main water supply tube 8110, and the like. Thepressure in the main water supply tube 8110 is read by the sensor 8013every time after a predetermined period, and is indicated on a meter.When the automatic/manual change-over switch has been set to the manualside, a voltage and a current instructed by the operator are transferredto the motor controller 8002.

To effect the control operation through the operation board 8011, first,the automatic/manual change-over switch is set to the manual side. Then,a stop signal is outputted to the controller 8001 and no instruction issent from the controller 8001. Next, upon seeing the pressure in themain water supply tube 8110 indicated on the meter, a voltage and acurrent to the motor 8102 for driving the pump 8012 are judged and areset using a setting dial. When an emergency stop switch is depressed,furthermore, a stop signal is sent to the motor controller 8002 to stopthe pump 8012. Thus, the pump 8012 is manually controlled.

Described below is the multi-master operation in the case when theoutputs are simultaneously produced from the controller 8001 and theoperation board 8011. When packets are simultaneously produced from thecontroller 8001 and the operation board 8011, arbitration is necessary.

According to ISO11898, the bus is driven when the logic 0 is outputtedto the bus but is not driven when the logic 1 is outputted thereto.Therefore, when the logic 0 is outputted even by a single node, the busas a whole is driven to exhibit the logic 0. That is, the logic 0 takesa precedence. It is further a prerequisite that the drive mode has beentransferred to all nodes within a time of transferring a bit. Uponobserving the mode of the bus at the end of the bit transfer time,therefore, the transfer of data of a bit ends within the time oftransferring the bit. Even when no data is being transmitted, every nodeis observing the value of the bus and can recognize the end of thepacket. When the transmission of packets is started from a plurality ofnodes, therefore, there can be learned the timings for transmitting thepackets. Therefore, the packets are necessarily transmittedsimultaneously.

From the foregoing, the arbitration is realized in the following manneraccording to ISO11898. The packet has a structure as shown in FIG. 4(c)and in which the order of priority of packet is set to the header 7001,and the packets having the same order of priority are neversimultaneously transmitted from different nodes. When the header 7001 isbeing transmitted, therefore, the node transmitting the packet comparesthe value which it has outputted to the bus with the mode of the bus atthe time when the mode of the bus is observed while a bit is beingtransferred. When they are not in agreement (the node has outputted thelogic 1 and the bus is in a mode of being driven), the node interruptsthe transmission of the packet and simply observes the mode of the busuntil it becomes possible to transmit the next packet.

Thus, when the transfer of the header 7001 ends, only the node that hastransmitted the packet having the highest order of priority remains, andthe other nodes all observe the mode of the bus. Arbitration is thuscarried out.

FIG. 19 is a diagram of shift for explaining the arbitration in thesystem.

(a) The order of priority of a packet in the water supply and sewagesystem is higher when it is that of an instruction by a man through theoperation board 8011 than that of an instruction from the controller8001. Therefore, the first two bits of the header 7001 of a packettransmitted from the operation board are set to be 00, and the first twobits of the header 7001 of a packet transmitted from the controller areset to be 01.

(b) The transmission of packets is started from the controller 8001 andthe operation board 8011; i.e., first bits are transmitted,respectively. The first bits of the headers 7001 from the controller8001 and the operation board 8011 are both 0, and both drive theelectric buses 20 a and 20 b. The electric buses 20 a and 20 b are bothdriven. The electric buses 20 a and 20 b are driven after having beenmonitored by the controller 8001 and the operation board 8011.Accordingly, the modes in which the buses are driven are in agreementwith each other.

(c) Then, the second bits are transferred. The second bit of thecontroller 8001 is 1 which does not drive the electric bus 20 a. Thesecond bit of the operation board 8011 is 0 which drives the electricbus 20 b. Therefore, the optical bus-bridging device 10 a assumes themode of the bus drive mode, the optical bus-bridging device 10 b assumesthe mode of the optical output mode, and the electric buses 20 a and 20b both assume the mode in which they are driven. In the controller 8001,therefore, driving of the bus is not in agreement with the mode of thebus 20 a. In the operation board 8011, on the other hand, driving of thebus is in agreement with the mode in which the bus 20 b is driven.

(d) Accordingly, the controller 8001 no longer transmits the packet butassumes the receiving mode, while the operation board 8011 continues totransmit the packet.

As described above, despite the packets are transmitted from both thecontroller 8001 and the operation board 8011 in accordance with thestandard of ISO11898, the operation board 8011 wins the arbitration;i.e., the data 7002 is transmitted, and the instruction from theoperation board 8011 takes precedence. Accordingly, the controller 8001no longer operates, and the automatic operation is quickly changed overto the manual operation.

When the two electric buses to which the nodes are connected are to beconnected through optical fibers, as described above, opticalbus-bridging devices having a mode shifting function are provided amongthe optical fibers and the electric buses. When the two buses are notdriven simultaneously, the optical bus-bridging devices assume thestandby mode to observe the modes of the buses of their own sides. Whenthe driven mode of one bus or two buses is observed, the twobus-bridging devices separately assume the optical output mode forsending an optical output to the optical fiber and the bus drive modefor sending an electric output to the electric bus, avoiding the“deadlocked situation” or the “misjudged situation” that occurs whenboth of them assume the optical output mode. Accordingly, the modes inwhich the two electric buses are driven can be properly brought intoagreement.

Moreover, the non-observation mode is passed through when the bus drivemode shifts to the standby mode, and a transient condition is noterroneously regarded to be that the bus is being driven, as a result ofnot observing the mode of the bus from the ON mode of the bus until itis stabilized in the OFF mode.

This makes it possible to constitute a system for substituting part ofthe network with optical fibers and, hence, to provide a network systemfeaturing excellent resistance against noise in the field at a low cost.Moreover, the buses driven by a plurality of nodes are arbitrated inaccordance with the standards of ISO11898, making it possible to bringthe modes of the buses into agreement and to easily constitute amulti-master system.

Next, described below is another method of bringing the modes in whichthe buses are driven into agreement.

FIG. 21 illustrates the constitution of an optical bus-bridging circuit.The communication system to which the optical bus-bridging device isadapted has been shown already in FIG. 1. The optical bus-bridgingdevices 10 have the same structure in the system A and in the system B,and are, hence, described without distinction depending upon thesystems. As required, furthermore, a sign of the signal line is added inparenthesis to the end of the name of the signal.

The optical bus-bridging device 10 is constituted by the bus drivercircuit 11, the state-of-the-bus judging circuit 14 and thephoto-electric converter 13. The bus driver circuit 11 is connected tothe bus 20 for transmitting signals transferred by the voltage, becausetransfer medium is a bus, hot voltage transferred by voltage, asdescribed already, and sends a signal to the bus to change the mode ofthe bus 20 or to receive a signal from the bus 20. The photo-electricconverter 13 is connected to the optical fiber 50 which transfers thelight, and converts the transfer medium from the voltage to the light orfrom the light to the voltage. The state-of-the-bus judging circuit 14receives a signal representing the mode of the bus 20 from the busdriver circuit 11 and a signal representing the mode of the opticalfiber from the photo-electric converter 13, determines the mode whichshould be assumed by the bus 20, and sends to the bus driver circuit 11a signal representing the determined mode that should be assumed by thebus 20.

The bus driver circuit in the optical bus-bridging device is the same asthe one shown in FIGS. 3 and 4, and the photo-electric converter 13 isthe same as the one shown in FIGS. 5 and 6.

FIG. 21 illustrates the internal constitution of the state-of-the-busjudging circuit 14. The state-of-the-bus judging circuit 14 isconstituted by a drive condition judging circuit 211, a time countercircuit 212, a time setting circuit 213 and a clock generating circuit214, receives a signal RxD_N outputted from the bus driver circuit 11through a signal line 131, receives a signal DOUT outputed from thephoto-electric converter 13 through a signal line 132, and outputs asignal TxD_N to the bus driver circuit 11 through a signal line 134. Thedriving condition judging circuit 211 outputs a counter reset signalRESET 2115 for resetting a counter in the time counter circuit 212, andthe time counter circuit 212 outputs a counter carry signal CARRY 2121to the drive condition judging circuit 211. The time setting circuit 213outputs a preset count PRESET 2132 to the time counter circuit 212. Theclock generating circuit 214 supplies a clock signal CK 2141 to thedrive condition judging circuit 211 and to the time counter circuit 212.

The drive condition judging circuit 211 determines the mode which shouldbe assumed by a bus 30 depending upon the signal RxD_N (131)representing a mode of the bus 30 to which it is connected and thesignal DOUT (132) representing the mode of the bus 20 of the opposingside obtained through the optical fiber 50, and produces a signal TxD_N(134) for driving the bus 30. In the bus driver circuit 11 as shown inFIG. 3, furthermore, the output of the transmitter 1102 is inputted tothe receiver 1101. Therefore, the mode of the bus 20 determined by thestate-of-the-bus judging circuit 12 is directly outputted. Accordingly,the signal inputted to the optical bus-bridging device 10 of theopposing side from the optical bus-bridging device 10 having the sameconstitution through the optical fiber 50, is directly sent to theoptical bus-bridging device 10 of its own side. Therefore, the opticalbus-bridging devices 10 discriminate, by using the time counter circuit212, whether the received signals are the signals sent by the opticalbus-bridging devices 10 themselves by feedback.

FIG. 29 illustrates the state transition graph of the state-of-the-busjudging circuit 14. The state-of-the-bus judging circuit 14 assumes fourmodes 0 to 3. The input conditions include a signal RxD_N (131)outputted from the receiver 1101 in the bus driver circuit 11, a signalDOUT (132) sent from the optical fiber 50 and converted into an electricsignal through the photo-electric converter 13, and a counter carrysignal CARRY 2121 from the time counter circuit 212. A signal TxD_N(134) is outputted and is fed to the transmitter 102 in the bus drivercircuit 11. The modes 0 to 3 are shifted from the one to the otherdepending upon the input conditions at the time when the clock signalCK2141 rises. The signal TxD_N (134) inputted to the transmitter 1102becomes 0 in the mode 1 only. Described below is the shift of the mode.In the initial mode, i.e., when the buses 20 a and B 20 b both remainturned OFF, both the signal RxD_N (131) output from the receiver 1101and the signal DOUT (132) outputted from the photo-electric converter 13assume 1, and the state-of-the-bus judging circuit 14 remains in themode 0. When the bus A 20 a is turned ON, the signal RxD_N (131) becomes0, and the mode is shifted to the mode 2. In the mode 2, the signalTxD_N (134) continues to assume 1, and the mode 2 is maintained as faras the signal RxD_N (131) is 0. When the bus A 20 a is turned OFF andthe signal RxD_N (131) assumes 1, the mode is shifted to the mode 3.Here, the time counter circuit 212 operates. The mode 3 is maintaineduntil a predetermined time elapses, i.e., so far as the counter carrysignal CARRY 2121 is 0. This makes it possible to wait for until themode of the bus B 20 b is reflected. When the signal DOUT (132) is 1 atthe time when the counter carry signal CARRY 2121 is 1, i.e., when thebus B 20 b is turned OFF, the mode is shifted to the mode 0. When thesignal DOUT (132) is 0 at the time when the counter carry signal CARRY2121 is 1, i.e., when the bus B 20 b is turned ON, the mode is shiftedto the mode 1. The mode 2 is maintained while the signal DOUT (132) is0, and the signal TxD_N (134) is set to 0 to drive the bus A 20 a so asto assume the ON mode. When the signal DOUT (132) becomes 1, the mode isshifted to the mode 0. The mode is shifted to the mode 1 even when thesignal RxD_N (131) is 1 in the mode 0 and the signal DOUT (132) becomes0.

FIG. 22 illustrates the constitution of the driving condition judgingcircuit 211. The driving condition judging circuit 211 is constituted bya synchronizing circuit 2111, a drive signal generating circuit 2112, apower-on reset generating circuit 2113 and an OR gate 2114. Thesynchronizing circuit 2111 receives the signal RxD_N (131) and thesignal DOUT (132) and produces a synchronizing signal RDS (21111) and asignal DOUTS (21112). The drive signal generating circuit 2112 receivesa synchronizing signal RDS (12111), a signal DOUTS (12112), a signalDOUT (132), a counter carry signal CARRY (2121) and a power-on resetsignal PONRES (21131) from the power-on reset generating circuit 2113,and produces a signal TxD_N (134). The power-on reset generating circuit2113 outputs the power-on reset signal PONRES (21131) for apredetermined period of time when the power source of the opticalbus-bridging device 10 is turned on, and resets the latches in the drivesignal generating circuit 2112 and in the time counter circuit 211. TheOR gate 2114 forms a counter reset signal RESET (2115) by taking aninverted OR of the power-on reset signal PONRES (21131) and asynchronizing signal RDS (21111). That is, the counter reset signalRESET (2115) is outputted during the power-on resetting period andduring the period in which synchronizing signal RDS (21111) is 1.

FIG. 23 is a diagram illustrating the constitution of the synchronizingcircuit 2111. The synchronizing circuit 2111 is constituted by fourlatches that fetch the data in synchronism with the clock signal CK(2141). The signal RxD_N (131) and the signal DOUT (132) change inasynchronism with the clock signal CK (2141), and are passed through twostages of latches, respectively, in order to obtain a synchronizingsignal RDS (21111) and a signal DOUTS (21112) that change in synchronismwith the clock signal CK (2141).

FIG. 24 illustrates the constitution of the drive signal generatingcircuit 2112. The drive signal generating circuit 2112 is constituted bya set/reset flip-flop 21121, a set condition circuit 21122 and an ORgate 21123, and the inverted output of the flip-flop 21121 is a signalTxD_N (134). The OR gate 21123 forms a flip-flop reset signal by takingan inverted OR of the power-on reset signal PONRES (21131) and thesignal DOUT (132). That is, the flip-flop 21121 is reset and the signalTxD_N (134) becomes 1 during the power-on resetting period and duringthe period in which signal DOUT (132) is 1.

FIG. 25 is a diagram illustrating the operation of the set conditioncircuit 21122, and in which DOUTS (t) represents a value of the presentsignal DOUTS (21112) and DOUTS (t-1) represents a value of the signalDOUTS (21112) of one clock before. That is, the flip-flop (21121) is setand the signal TxD_N (134) becomes 0 when the value of the signal DOUTS(21112) of one clock before is 0, the value of the present signal DOUTS(21112) is 1 and the value of the synchronizing signal RDS (21111) is 1,or when the value of the counter carry signal CARRY (2121) is 1 and thevalue of the synchronizing signal RDS (21111) is 1.

FIG. 26 is a diagram illustrating the operation of the time countercircuit 212. The time counter circuit 212 includes a counter circuitwhich effects the count-up in synchronism with the rise of the clockwhen the condition holds true. The counter circuit, however, is agenerally employed one and is not described here. In the presentinvention, the counter circuit comprises a 6-bit register and countsvalues from 0 up to 63. When the reset input RESET (2115) is 1, thecounter assumes a value i (i=0 to 62) of preset count PRESET (2132) asthe clock signal CK (2141) rises, and the value of the counter isincremented for every rise of the clock signal CK (2141) so far as RESET(2115) is 0. When the value of the counter is 63, the counter carrysignal CARRY (2121) becomes 1. A series of the operations is shown in atime chart of FIG. 27.

FIG. 28 illustrates the constitution of the time setting circuit 213.The time setting circuit 213 is constituted by a 6-bit setting switch2131 and a pull-up resistor 2133, and outputs 0 for each of the bitswhen the switch is closed and outputs 1 when the switch is opened.

Next, described below is the operation of the case when the opticalbus-bridging device shown in FIG. 20 is adapted to the communicationsystem of FIG. 1.

First, concretely described below is how to obtain the right fortransmission when a plurality of nodes are to transmit messages. The busA 20 a is turned ON when the node 30 a turns the bus A 20 a OFF (withoutdriving it) based on a first bit that constitutes an identifier totransmit a message and when the node 30 b drives the bus to turn it ONbased on a first bit which constitutes an identifier to transmit amessage. Next, the node 30 a that has attempted to turn the bus A 20 aOFF monitors the mode of the bus A 20 a, detects the mode of the bus A20 a that is turned ON, i.e., detects the fact that it could not turnthe bus A 20 a OFF. Similarly, the node 30 b monitors the mode of thebus A and detects the fact that it could turn the bus A 20 a ON. This isdone for every bit constituting the identifier, and either the node 30 aor the node 30 b obtains the right for transmission. This method hasbeen described in detail in a literature quoted in the prior art. Theoperation of the node from outputting a bit which is in an identifier upto the detection of the mode of the bus is executed within a time oftransmitting a bit. In the operation of the system described below, apoint at which the mode of the bus is detected by the node is calledsampling point.

FIG. 31 illustrates a mode where the node (30 a or 30 b) connected tothe bus A 20 a drives the bus A 20 a so as to be turned ON in a giventransfer cycle but does not drive the bus in the next transfer cycle,and the nodes (30 c and 30 d) connected to the bus B 20 b do not drivethe bus B 20 b during this period.

When the node 30 a turns the bus A 20 a ON, the signal RxD_N (131)outputted from the receiver 1101 in the bus driver circuit 11 a of theoptical bus-bridging device 10 a changes from the logic 1 to the logic0. Through the electric-photo conversion unit E/O 1302 in thephoto-electric converter 13, the signal RxD_N (131) is transmitted, as alevel of a signal OOUT (501), to the optical bus-bridging device 10 bthrough the optical fiber 50 after a delay time due to opticaltransmission. The signal OIN 502 inputted to the photo-electricconverter 13 b of the optical bus-bridging device 10 b changes from 1 to0, and a signal DOUT (132) converted through the photo-electricconversion unit O/E 1301 is outputted. The state-of-the-bus judgingcircuit 14 b in the optical bus-bridging device 10 b sends a signalTxD_N (134) of the logic 0 to the transmitter 1102 in the bus driver 11b since the signal OIN (502) has changed to the logic 0 while the bus ofits own side is turned OFF, i.e., while the bus B 20 b is in the OFFmode, and drives the bus B 20 b to assume the ON mode. Upon receivingthe logic mode of the transmitter 1102, on the other hand, the receiver1101 in the optical bus-bridging device 10 b sends the signal RxD_N(131) of the logic 0 to the optical fiber 50 through the photo-electricconverter 13 b so that it is transmitted to the optical bus-bridgingcircuit 10 a. However, since the bus A 20 a has been turned ON already,the state-of-the-bus judging circuit 14 a in the bus-bridging circuit 10a does not change the signal TxD_N (134) inputted to the transmitter1102 of the bus driver circuit. The nodes 30 a and 30 b fetch the modeof the bus A 20 a at the end of the bit transmission time, i.e., at thesampling point 1, and detect the ON mode of the bus.

When the nodes 30 a and 30 b do not drive the bus A 20 a in the next bittransfer cycle, the signal RxD_N (131) outputted from the receiver 1101of the bus driver circuit 11 a in the optical bus-bridging device 10 achanges from the logic 0 to the logic 1. Through the electric-photoconversion unit E/O 1302 of the photo-electric converter 13 a in theoptical bus-bridging device 10 a, the signal RxD_N (131) is transmitted,as a level of the signal OOUT (501), to the optical bus-bridging device10 b through the optical fiber 50, and the signal DOUT (132) outputtedfrom the photo-electric conversion unit O/E 131 of the photo-electricconverter 13 b in the optical bus-bridging circuit 10 b changes from thelogic 0 to the logic 1. Then, the counter reset signal RESET (2115) inthe optical bus-bridging device 10 b changes from the logic 1 to thelogic 0, and the time counter circuit 212 starts the counting operation.The state-of-the-bus judging circuit 14 b in the optical bus-bridgingdevice 10 b outputs the signal TxD_N (134) of the logic 1 to thetransmitter 1102 in the bus driver circuit 11 b since the signal DOUT(132) is outputted while it is driving the bus B 20 b, and no longerdrives the bus B 20 b. Then, the mode of the bus B 20 b changes into theOFF mode. The OFF mode of the bus B 20 b is fed back to the opticalbus-bridging device 10 a through the bus driver circuit 11 b of theoptical bus-bridging device 10 b, the photo-electric converter 13 b andthe optical fiber 50. The mode of the bus B 20 b that is fed back isjudged depending upon the fact that the counter carry signal CARRY(2121) from the time counter circuit 212 in the optical bus-bridgingdevice 10 a has changed into the logic 1. In this example, the bus A 20a has been turned OFF already, and the state-of-the-bus judging circuit14 a in the optical bus-bridging device 10 a does not change the inputsignal TxD_N (134). The nodes 30 a and 30 b fetch the mode of the bus A20 a at the end of the time for transferring the bit, i.e., at asampling point 2, and detects the OFF mode of the bus.

FIG. 32 illustrates a mode where the node (30 a or 30 b) connected tothe bus A 20 a drives the bus A 20 a to assume the ON mode in a giventransfer cycle but does not drive the bus in the next transfer cycle,and the nodes (30 c and 30 d) connected to the bus B 20 b do not drivethe bus B 20 b in the first transfer cycle but either one of the nodesdrives the bus B 20 b in the next transfer cycle.

When the bus A 20 a is turned ON, the signal RxD_N (131) of the receiver1101 of the bus driver circuit 11 a in the optical bus-bridging device10 a changes from the logic 1 to the logic 0. This signal is thentransmitted, as a level of the signal OOUT (501) of the photo-electricconverter 13 a in the bus driver circuit 11 a, to the opticalbus-bridging device 10 b through the optical fiber 50 after a delay timeof optical transmission. The photo-electric converter 13 b in theoptical bus-bridging device 10 b receives the signal OIN (502) which haschanged from the logic 1 to the logic 0, and a signal DOUT (132) of thelogic 0 is outputted from the photo-electric converter 1301. Thestate-of-the-bus judging circuit 14 b in the optical bus-bridging device10 b outputs a signal TxD_N of the logic 0 to the bus driver circuit 11b since the signal DOUT (132) has changed into the logic 0 while the busof its own side is in the OFF mode, i.e., while the bus B 20 b is in theOFF mode, and drives the bus B 20 b to assume the ON mode. The ON modeof the bus B 20 b is fed back to the optical bus-bridging device 10 athrough the bus driver circuit 11 a of the optical bus-bridging device10 a, the photo-electric converter 13 b and the optical fiber 50. Atthis moment, the bus A 20 b has been turned ON already and, hence, thestate-of-the-bus judging circuit 14 a in the optical bus-bridging device10 a does not change the signal TxD_N (125) of the bus driver circuit 11a. The nodes 30 a and 30 b fetch the mode of the bus A 20 a at the endof the time for transferring the bit, i.e., at a sampling point 3, anddetects the ON mode of the bus. The operation of the first transfercycle is quite the same as that of FIG. 30.

When the nodes 30 a and 30 b do not drive the bus A 20 a in the nexttransfer cycle, the signal RxD_N (131) of the bus driver circuit 11 a inthe optical bus-bridging device 10 a changes from the logic 0 to thelogic 1. This signal is transferred, as a level of OOUT (501) of thephoto-electric converter in the optical bus-bridging device 10 a, to theoptical bus-bridging device 10 b through the optical fiber 50, and thephoto-electric converter 13 b in the optical bus-bridging device 10 boutputs a signal OIN (502) that has changed from the logic 0 to thelogic 1 as a signal DOUT (132) of the photo-electric conversion unit O/E1301. When the signal RxD_N (131) of the bus driver circuit 11 a in theoptical bus-bridging device 10 a changes from the logic 0 to the logic 1due to the bus A 20 a that has changed into the OFF mode, the counterreset signal RESET (2115) of the drive condition judging circuit 211 inthe optical bus-bridging device 10 a changes from the logic 1 to thelogic 0, and the time counter circuit 212 starts the counting operation.

Upon receiving a signal DOUT (132) from the photo-electric converter 13a due to a signal transmitted from the optical bus-bridging device 10 a,the state-of-the-bus judging circuit 14 a in the optical bus-bridgingdevice 10 a changes the logic of the signal TxD_N inputted to the busdriver circuit 11 b to 1 since the signal DOUT (132) is assuming 1 whileit is driving the bus B 20 b, and no longer drives the bus B 20 b. Inthis example, however, the nodes 30 c or 30 d is already driving the busB 20 b and, hence, the bus B 20 b remains in the ON mode. Therefore, thesignal RxD_N (131) of the logic 0 is continuously outputted from the busdriver circuit 11 b of the optical bus-bridging device 10 b. Hence, thesignal OOUT (501) which is outputted from the optical bus-bridgingdevice 10 b to the optical fiber 50 maintains the logic 0. In theoptical bus-bridging device 10 a, on the other hand, the counter carrysignal CARRY (2121) of the time counter circuit (212) assumes the logic1 after the passage of a predetermined period of time, and thestate-of-the-bus judging circuit 14 a of the optical bus-bridging device10 a that has received this signal sends an input signal TxD_N (134) ofthe logic 0 to the bus driver circuit 11 a to drive the bus A 20 a so asto assume the ON mode. Thereafter, the nodes 30 a and 30 b fetch themode of the bus A 20 a at a sampling point 4 and detect the ON mode ofthe bus, i.e., detect that the mode of the bus B 20 b is properlyreflected by the bus A 20 a.

Next, described below is how to set the time setting circuit (213) inthe optical bus-bridging devices 10 a and 10 b. It is now presumed thatthe optical fiber 50 has a length of 500 m, the propagation speed is 5nsec/m, the oscillation frequency of the clock generating circuit 214 is10 MHz, and the sum of passage times of the optical bus-bridging devices10 a, 10 b, i.e., the delay time of the bus driver circuits 11 a, 11 b,the delay time of the state-of-the-bus judging circuits 14 a, 14 b, andthe delay times of the photo-electric conversion circuits 13 a, 13 b is500 nsec. In this case, the time that should be counted by the timecounter circuit 212 is the sum of the round-trip transfer delay throughthe optical fiber and the passage time through the bus-bridging devices,i.e., 500×5×2+500 nsec=5.5 μ sec. The counter is capable of counting upto 6 bits, i.e., up to 63, and is counted up every time after 100 nsec.Therefore, if PRESET (2132) has been initially set to 9, the countercarry signal CARRY (2121) assumes the logic 1 after 5.5 μ sec havepassed from when RESET (2115) has assumed the logic 0. When the opticalfiber 50 has a length of 250 m, then, the time is 3.0 μ sec, and theinitial setting will be 34. It is thus allowed to set a value inproportion to the length of the optical fiber 50, and correct transferis accomplished even when the optical fibers have dissimilar lengths.

When the lengths of the buses A 20 a and B 20 b, i.e., the electrictransfer times, are too long to be neglected with respect to the lengthof the optical fiber 50 or the optical transfer time, the electrictransfer time must be added to a value set by the time setting device213. It is now presumed that the buses A 20 a and B 20 b have a lengthof 100 m, respectively, the propagation speed is 5 nsec/m, the opticalfiber 50 has a length of 300 m, the propagation speed is 5 nsec/m, theoscillation frequency of the clock generating device 214 is 10 MHz, andthe sum of the passage times of the optical bus-bridging devices 10 aand 10 b, i.e., delay times of the bus driver circuits 11 a, 11 b, thedelay times of the state-of-the-bus judging circuits 14 a, 14 b, and thedelay times of the photo-electric conversion circuits 13 a, 13 b is 500nsec. In this case, the time to be counted by the time counter circuit212 is the sum of the round-trip transfer delay of the optical fiber,passage time of the bus-bridging devices and the round-trip transfertime of either the bus A or the bus B, i.e., 300×5×2+500+100×5×2nsec=4.5 μ sec, and the initial setting will be 19. Thus, a valueproportional to the length of the electric bus is added to a valueproportional to the length of the optical fiber to properly transfer thesignals even when the electric bus is long.

By using the optical bus-bridging device of the present invention asdescribed above, it is allowed to minimize the time for transferring abit, and the transfer time of the data processing system can beshortened.

The above-mentioned embodiment has dealt with the case where the messagehas an identifier and the degree of priority is determined by theidentifier. The invention, however, is in no way limited thereto only,and the transfer time of the whole system can be similarly shortenedeven when the data are to be simply transferred finding a wide range ofapplications.

As described above, the mode of the bus of the opposing side can beproperly judged by the state-of-the-bus judging circuit using a timecounter circuit, and the same mode of the buses can be maintained onboth sides of the bus-bridging devices within a time of transferring abit.

Even when the buses are simultaneously driven by a plurality of nodes,there can be realized a data processing system capable of transferringdata at high speed using optical fibers.

What is claimed is:
 1. An optical communication method in which themodes of two electric buses connected through optical fibers, aplurality of nodes being connected to said buses, are brought intoagreement, comprising the steps of: observing the modes of said electricbuses and the modes of said optical fibers while said electric buses arenot being driven (OFF mode); maintaining optical output from the busesthat are being driven to said optical fibers while one or both of saidelectric buses are being driven (ON mode) by the nodes connectedthereto; maintaining electric output to the electric bus of the side towhich light is inputted to drive the bus without observing the modes ofsaid buses while light is being inputted from said optical fibers; andstopping said optical and electric outputs to restrict said electricbuses from being driven when the buses are no longer driven by saidnodes.
 2. An optical communication method according to claim 1, whereinat the time of being shifted to the non-driven mode by no longerproducing the electric output, observation to said bus is suspended foronly a predetermined period of time.
 3. An optical communication methodaccording to claim 1 or 2, wherein when both of said electric buses aredriven by the respective nodes and when optical outputs are sent to saidoptical fibers from both sides, one side discontinues the production ofsaid optical output and produces said electric output only.
 4. Opticallinking devices installed among optical fibers for connecting twoelectric buses in order to bring the modes of the two electric busesinto agreement, comprising a means for executing a standby mode toobserve the modes of the buses and the modes of the optical fibers whensaid electric buses are not being driven (OFF mode), an optical outputmode shifted from said standby mode to produce an optical output to saidoptical fibers when said electric buses are driven (ON mode) by thenodes to which they are connected, a bus drive mode for producing anelectric output to the electric bus of its own side when an opticalinput is received from said optical fibers, and a non-observation modewhich, when the buses are no longer driven by said nodes, inhibits theobservation of the modes of the buses for a predetermined period of timeat the time when said bus drive mode is shifted to said standby mode,wherein said means changes over these modes depending upon the modes ofthe buses.
 5. Optical linking devices according to claim 4, furthercomprising a mode shift-signal setting means for shifting one of the twooptical bus-bridging devices into the bus drive mode when the twoelectric buses are driven by the nodes, and when the two opticalbus-bridging devices provided on both sides of said optical fibers aresimultaneously changed over to said optical output mode.
 6. An opticalcommunication system comprising electric buses having two electricalmodes, a plurality of nodes for outputting two-value data to saidelectric buses, optical linking devices having means for convertingelectric signals into optical signals and means for converting opticalsignals into electric signals, and optical fibers for connecting saidtwo electric buses together via said optical linking devices, wherein:fiber include two optical fibers through which said optical linkingdevices execute optical output and optical input separately in order totransmit the modes of said electric buses in two directions; saidoptical linking devices have a function for observing the ON/OFF mode ofsaid electric buses and the presence/absence of optical input from saidoptical fibers, for producing optical outputs to said optical fiberswhen said electric buses are in the ON mode, and for producing anelectric output to the electric bus of its own side when an opticalinput is received from said optical fibers, and a function for haltingthe optical output of one side and for producing said electric outputonly when said two optical fibers have simultaneously produced saidoptical outputs giving rise to the occurrence of an optical loopsituation; and said electric buses are driven for each of thetransmission cycles depending upon the ON/OFF of a bit data from saidnode, the driven modes of the two electric buses are brought intoagreement via said optical fibers and said optical linking devices onboth sides thereof, and after the modes have been brought intoagreement, said nodes execute the sampling of said electric buses.
 7. Anoptical communication system according to claim 6, wherein when saiddata are simultaneously outputted from said plurality of nodes to saidelectric buses, the modes of said buses are necessarily determined to bea preferential mode, and every node compares the mode in which it hasproduced an output to said electric bus with the mode of said electricbus and determines whether the data be continuously outputted to saidelectric bus or not.